1. Technical field
This invention relates to an impact driver which is used for loosening screws and nuts corroded with rust to cause difficulty in loosening by utilizing shocks, or for tightening screws etc. securely in a final stage.
2. Prior art
In a screw fastening structure, it sometimes occurs that a screw sticks to its fitting member or a nut due to rust with an elapse of time, and the screw can not be removed by an ordinary driver or spanner in such a case.
To cope with this kind of trouble, the screw is removed by using a chisel or a gas cutting device when the screw or screw hole can be broken, but it is removed by applying shocks on a driver head using a hammer when the screw hole or screw fitting member is to be utilized again.
In recent years, an impact driver is proposed whereby a screw can be removed without being broken not only by applying shocks on its head but by converting a shock of hammer into a twisting torque utilizing a cam mechanism to make a bit of the driver generate an instantaneous large torque.
Namely, in this impact driver; a chuck is fitted rotatably in a cylindrical casing, a cam mechanism for converting an axial movement of the casing into a rotational movement of the chuck is installed between the casing and the chuck, and the bit of the chuck is turned with an instantaneous large torque applied on a top wall of the casing with a hammer by an operator.
However, since the operator strikes the top wall with the driver in one hand and the hammer in the other, the operator sometimes damages the fitting member or strikes his finger due to faulty aim.
Further, a large torque loss will be produced because the operator strikes the entire casing having a large mass and the shock is transmitted from the casing throug the cam mechanism to the chuck.